Relevance of the organic carbon to clay ratio as a national soil health indicator

Summary

This peer-reviewed study evaluated the suitability of the SOC/clay ratio as a national-scale soil health indicator in mainland France, following its selection for the European Soil Monitoring Law. The authors found that the SOC/clay ratio was a poor predictor of actual soil structural quality (bulk density and aggregate stability) and was strongly confounded by soil pH, with acidic soils systematically misclassified as healthy and alkaline soils as unhealthy. The study questions the appropriateness of the proposed 1/13 threshold and the indicator's applicability across diverse pedoclimatic contexts and SOC stabilisation mechanisms.

UK applicability

Given that the SOC/clay indicator was previously tested in England and Wales before this French study, the UK findings may already be known; however, this study's evidence of soil pH confounding and poor structural quality prediction should inform UK policy makers considering adoption of this indicator under European harmonisation frameworks.

Key measures

SOC/clay ratio, SOC/(silt <20 µm + clay) ratio, soil bulk density, aggregate stability, soil pH, land use classification, soil type

Outcomes reported

The study evaluated whether the SOC/clay ratio and its variant SOC/(silt <20 µm + clay) were relevant indicators of soil structural quality at the national scale in mainland France, using bulk density and aggregate stability as structural quality benchmarks. The research also assessed how land use, soil type, and soil pH affected the SOC/clay indicator's classification of soil health against the proposed European threshold of 1/13.

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